This invention relates to an ATM (Asynchronous Transfer Mode) signal switching apparatus and, in particular, to a switching reference in switching ATM virtual paths (VP's: Virtual Paths) and ATM virtual channels (VC's: Virtual Channels) in order to obtain high reliability for the ATM network formed by the signal switching apparatus. This invention relates also to a device and a method for transferring failure information as the switching reference in the ATM network.
For convenience of description, VP switching alone will be described hereinafter. VC switching is carried out in a manner similar to the VP switching.
In the prior art, the following methods are known.
(1) VP AIS (Alarm Indication Signal) Based Switching Method
Upon occurrence of a line failure, a path failure (for example, an STS-1 path failure), or a failure in a VP itself, a VP AIS is inserted downstream. At a receiving end, detection of the VP AIS is used as a i.e., switching reference (switching trigger).
The above-mentioned method is described also in T1.X1.5/96-044 Alcatel Contribution "ATM VP Uni-Ring Fault Reconfiguration". An AIS based switching method itself has been generally practiced in a SONET (Synchronous Optical Network). In this method, the path failure is transmitted via a Path AIS and the detection thereof is used as the switching trigger. A UPSR (Unidirectional Path Switched Ring) of the SONET adopts this method (GR-1400-CORE Issue 1, Bellcore March 1994).
(2) PM (Performance Monitoring) Based Switching Method
By PM carried out at a VP terminating point, the status of the VP is detected.
It is used as a switching reference (switching trigger). This method is also described in T1.X1.5/96-044 Alcatel Contribution"ATM VP Uni-Ring Fault Reconfiguration".
(3) Switching Message Based Switching Method
There is a method of switching in response to notification or delivery of a switching message. For example, in a SDH (Synchronous Digital Hierarchy) network, protection using K1 and K2 is widely used. As an application of the above-mentioned scheme to an ATM, there is a method of delivering the message by the use of OAH (Operation: Administration and Maintenance) cells.
JP-A No. 157942/1992 entitled "Fault Announcing Cell Generating Apparatus" discloses a method of regenerating fault announcing cells for notification. In T1.X1.5/96-044, T1X1.5/96-087, and T1S1.5/96-087, Alcatel contribution entitled "ATM VP Uni-Ring Fault Reconfiguration" a method of transmitting VP Segment Status/Link Status messages by the use of regenerated cells is proposed.
JP-A No. 235983/1993 entitled "Virtual Path Switching Apparatus and Switching Method" describes a technique of detecting a fault by path monitoring means, propagating a switching control signal on a spare channel to maintain a capacity, and thereafter performing the switching operation, although a practical propagation scheme is not described in detail.
JP-A No. 287539/1992 entitled "Alarm Transfer Method and Transfer System in an ATM Network" describes an alarm transfer method in which the failure is restricted to a link failure and all VPI's (Virtual Path Identifiers) accommodated in a link in question are dealt with. The VPI's are recognized from a VPI management table. For each of the VPI's, a cell containing alarm information is formed and delivered.
Problems in the switching operation triggered by the VP AIS are:
(1) It is difficult to insert the VP AIS for VP's up to 4096 at maximum (for VC's, much greater). In addition, insertion must simultaneously be carried out at a high speed.
(2) This means that new OAM cells are inserted. It is therefore necessary to newly assign the OAM cells to a transmission path band currently used. In those cases where user cells are completely lost, such as Line LOS (Loss of Signal), the insertion of the VP AIS as the OAM cells results in no problem. However, in those cases where the user cells are not lost, for example, in the case of B2 errors, the insertion of the VP AIS may suppress the band allotted to the user cells.
(3) The use of the OAM cells inevitably results in a disadvantage of slow message transmission. The insertion of the VP AIS requires at least 500 msec before the first insertion takes place. Subsequent insertion is carried out at a rate of 1 cell/1 second (GR-1248-CORE Issue 1, Bellcore August 1994).
Accordingly, when the switching operation is performed in response to the detection thereof, several seconds are required taking a VP AIS detection guard time into consideration. Thus, the switching operation is very slow.
Problems in the switching operation triggered by the VP PM are:
(1) Up to 30 VP PM cells are simultaneously monitored at maximum (GR-1248-CORE Issue 1, Bellcore August 1994). Thus, the objects to be switched are restricted in number to 30.
(2) The insertion of the VP PM cells is carried out at a rate of 1 cell/128 cells (depending upon the case, 1 cell/256 cells, 1 cell/512 cells, 1 cell/1024 cells) (GR-1248-CORE Issue 1, Bellcore August 1994). Therefore, switching by this approach requires a long switching time.
Problems in the switching operation in response to the switching message in the OAM cells are:
(1) Communication is required to transmit the message between nodes. This requires insertion of new OAM cells and complicated message processing for those OAM cells. As a result, a processing circuit has a corresponding increase in scale.
(2) Since the new QAM cells are inserted, these OAM cells are newly assigned to a transmission path band currently used. This may suppress the band allotted to the user cells. Generally, the number of bits required in generation of n cells for 4096 VP's is calculated by n.times.53.times.8.times.4096=n.times.1.7 Mbit.
In case of generation of n cells per one second, the band required is calculated as n.times.53.times.8.times.4096=n.times.1.7 Mbps.
(3) Delivery of the switching message requires message recognition and communication. This requires additional processing time which will result in slow switching.
The slow switching is considered as follows.
The insertion of VP PM cells prescribed in GR-1248-CORE is carried out at a rate of 1 cell/128 user cells. For low-frequency errors, small cell loss, and misinsertion, no trouble is caused if the switching operation takes a slightly longer time resulting in no serious influence imposed upon the application. On the other hand, upon occurrence of serious failure such as Line LOS, the switching operation must be performed as as soon as possible. Accordingly, a problem arises if the same approach is applied and the switching time is uniformly delayed.
The above-mentioned methods have critical disadvantages such as increased scale of the processing circuit related to the switching operation, longer processing time, suppression of the band allotted to the user cells, and slower switching.